1. Field of the Invention
This invention relates to the recovery of oil from subterranean reservoirs and more particularly concerns an improved process for recovering oil from porous reservoirs wherein carbon dioxide is used as a dense fluid displacement slug.
2. Description of the Prior Art
Enhanced recovery of petroleum by the injection of carbon dioxide into an input well and withdrawal of reservoir fluids from a producing well has long been known and practiced. Carbon dioxide has been used alone at high pressures wherein significant oil recoveries have been obtained. Such a process is described by Whorton et al. in U.S. Pat. No. 2,623,596. It was later discovered that the recovery of petroleum with carbon dioxide could be greatly increased if the carbon dioxide was used in slug form and driven through the reservoir by an aqueous drive fluid such as brine, water or carbonated water. A typical process in which carbon dioxide is driven through the reservoir by an aqueous fluid is fully described in Holm, U.s. Pat. No. 3,065,790. Next, Bernard et al. in U.S. Pat. No. 3,342,256 disclosed the improvement of an enhanced oil recovery process using carbon dioxide by incorporating a surface-active agent or surfactant prior to or with the carbon dioxide displacement slug. And Bernard in U.S. Pat. No. 3,529,668 disclosed a foam drive oil recovery process in which an aqueous surfactant solution is driven through the reservoir by alternating slugs of gas and aqueous drive fluids in certain critical proportions.
Copending application Ser. No. 767,529 filed Feb. 10, 1977 disclosed a carbon dioxide foam flooding process employing an alkali metal salt of an alkyl sulfocarboxylic acid as the foaming agent. Jacobs et al. disclose in U.S. Pat. No. 3,330,346 the use of an alkyl polyethylene oxide sulfate having between about 10 and about 16 carbon atoms in the alkyl group, such as the commercial detergent product marketed by Proctor & Gamble Company under the trademark "OK Liquid." However, aqueous solutions of these preferred foaming agents undesirably precipitate solids upon exposure to highly saline formation fluids.
It has been found that in spite of the wide variety of foaming agents known for foaming aqueous base media, the combination of carbon dioxide, brine, and formation constituents present many problems in the use of foaming agents in a displacement fluid in enhanced oil recovery processes. The mixture of carbon dioxide and formation fluids often constitutes an acidic and highly saline composition. Many foaming agents are not stable in acidic media and tend to decompose. Most of the foaming agents which are stable in low salinity acidic media become less stable or even decompose in highly saline acidic media. Although many foaming agents satisfactorily foam carbon dioxide at atmospheric pressure and low temperatures, in the low pH and high salinity conditions encountered in many formations, most foaming agents are rendered ineffectual. Furthermore, some carbon dioxide oil recovery processes have been conducted at high formation pressures, wherein the expansion of the carbon dioxide fluid is slight and the foaming agents tested have contributed very little to the efficiency of the process. There exists a need for an improved enhanced oil recovery process using carbon dioxide at high pressures especially in reservoirs of high salinity and low pH.
Accordingly, it is a primary object of this invention to provide an improved carbon dioxide enhanced oil recovery process by which greater quantities of oil can be produced.
Another object of this invention is to provide a high pressure carbon dioxide enhanced oil recovery process in which greatly enhanced areal sweep efficiencies of high salinity and low pH reservoirs are obtained.
Still another object of this invention is to more effectively utilize the carbon dioxide used in a carbon dioxide enhanced oil recovery process.
Other objects and advantages of this invention will become apparent to those skilled in the art from the following description.